StGit: stgit mail configuration options

The "stg mail" command uses a couple of configuration options that are completely undocumented, and don't fit well together.

"stgit.autobcc" seems to give a default value for the --bcc option.

"stgit.mail.prefix seems to give a default value for the --prefix option.

Shouldn't these both be stgit.* or stgit.mail.*? And shouldn't they be documented? Or maybe removed?

And I propse that the autobcc option should be a boolean which means that user.email should be used.